This report describes studies on the regulation of mineral metabolism and the pathophysiology of osteopenia in aging. These findings summarize investigations on: 1. The loss with age of bone mineral from rat femurs, measured by single photon absorptiometry. 2. Bone status of the aging female rat. 3. Mineral homeostasis and skeletal histology in the aged female rat. 4. Characterization of the human osteosarcoma cell line (CRL- 1427) as a model osteoblast-like human bone cell. 5. Regulation of bone resorption by a phenylalkylamine-sensitive calcium channel. 6. Biochemical mechanism of the age-associated decrease in vitamin D-sensitive intestinal calcium absorption. 7. Desensitization to PTH in Renal Cells from Aged Rats is Associated with a Decrease in PTH Receptor Binding Sites. 8. Desensitization to PTH in Renal Cells from Aged Rats Can Be Reversed by Parathyroidectomy of the Senescent Animal. 9. Preparation of cDNAs of Gs, Gi, Go and actin. 10. Pertussis Toxin Blocks the Action of alpha2-Adrenergie Hormones in Blunting the Response of Renal Cells to PTH. 11. PTH Regulation of Cytosolic Ca2+ in Proximal Tubules. 12. The PTH-Induced Increase in Cytosolic Ca2+ is Desensitized by PTH. 13. The PTH-Induced Increase in cAMP in OK Cells is Densensitized by PTH. 14. The Concentration of Ionized Ca in Serum is Unaltered in the Aged Male Rat. 15. Regulation of PTH-Independent Phosphate Uptake in Cultured Renal Cells (OK Cells) by Calcium. 16. PTH Enhances 25(OH)vit.D3 1-Hydroxylase Activity in Renal Cells.